WonderQuest with April Holladay, Article printer-friendly version

Cells that last a lifetime

Q: If I am 95 years old, do I have any cells in my body that were present when I was 3 years old? What about at conception? Also, could you expand on your earlier statement, "stem cells live a lifetime?" In a nutshell, I am curious if I am a completely "new" person or if there is anything "in" me that has been with me since birth...or conception.

A: Nope, we're not completely new people. In fact, most of our cells are a good seven to ten years old, as biologist Jonas Frisén, professor at the Karolinska Institute in Stockholm, reported in July 2005. For instance, we grow a new skeleton over about a 10-year period.

Lifetime cells: The photographer's new born has cortex neurons, heart muscle cells and eye lens cells she will still have the day she dies. Photos courtesy of Ernest F. and Wikipedia.

Moreover, at 95 we will have three kinds of cells we had since three years of age and, indeed, since before birth. They last a lifetime:

cerebral cortex neurons
heart muscle cells
cells in the inner portion of the crystalline lens of our eyes.

"I wouldn't necessarily single out cerebral cortex neurons as lasting a lifetime because it's probably true that most neurons in the entire adult brain were generated during development," emails neuroscientist Elizabeth Gould professor of psychology at Princeton.

The brain can repair and renew itself, but most of the cells in the cerebral cortex and the brain last a lifetime. We used to think that the cerebral cortex nerve cells remained unchanged. Now we know better. Brain cells, even in the cerebral cortex, renew themselves. In 1999, Frisén startled the neuroscience community by announcing he had found stem cells in the brain. Stem cells create new cells and, in this case, new brain cells.

Now, however, Frisén seems to have changed his thinking. "My estimate is that 100% of cortical neurons stay with us from birth," he emails.

That's probably true, but doesn't rule out the possibility that the brain also makes new cells. "Two very different types of cells live a life time. Hence, your brain has both fully differentiated neurons that live a lifetime, and 'stem cells' that continue to make new brain cells," clarifies researcher Gail W. Sullivan (retired assistant professor at the Cardiovascular Research Center in Charlottesville, Virginia).

The heart is still more controversial. Most heart muscle cells probably stay with us, from birth. Again, like the cerebral cortex, recently we believed heart muscle remained unchanged throughout a lifetime. However, cardiovascular researcher Piero Anversa has found stem cells in rat hearts and new heart muscle cells.

The crystalline lens in our eyes has innermost cells that last a lifetime. Lens cells are like skin cells in structure, but are totally unlike skin in lifespan. The skin renews itself daily; whereas the lens never loses cells. The lens does add more cells to the outside of the lens, but the oldest, inside ones have been there since before birth.

A fertilized egg divides to form inside eggs, which are embryonic stem cells. The stem cells can create any cell, for example, heart, brain or T-cell. Diagram courtesy of Mike Jones and Wikipedia, modified by author.

A fertilized egg divides to form inside eggs, which can create any cell, for example, heart, brain or T-cell. Diagram courtesy of Mike Jones and Wikipedia, modified by author.

Finally, there's the story of the immortal stem-cell DNA, which begins with the fertilized human egg, at conception. The fertilized egg divides repeatedly to form ten cells: the inside and outside cells. The inside cells divide more, and become the embryo.

Each inside cell is a stem cell that can form any of the 220 kinds of cells in the body — heart, brain, T-cell — whatever. These remarkable embryonic stem cells don't last a lifetime, but their DNA does.

An ordinary cell splits to form two daughter cells, and duplicates its DNA so each daughter cell gets a copy. But over a lifetime, as a cell reproduces perhaps thousands of times, duplication errors can build up, and cause trouble. The embryonic stem cell, consequently, doesn't do it this way, as Shahragim Tajbakhsh of the Pasteur Institute in Paris recently discovered.

The stem cell splits into two daughter cells (as does an ordinary cell), but instead of creating two ordinary cells, it forms another stem cell and a specialized cell (for example, a heart cell). Now, the crucial difference — the stem cell retains the original DNA strands, and makes a DNA copy only for the specialized cell. So, the DNA of the original fertilized egg lasts from conception to death.

Click for an animated demonstration of how an animal cell splits to form two daughter cells, courtesy of Cells Alive.

WonderQuest with April Holladay, Article printer-friendly version

Cells that last a lifetime

Lifetime cells: The photographer's new born has cortex neurons, heart muscle cells and eye lens cells she will still have the day she dies. Photos courtesy of Ernest F. and Wikipedia.

A fertilized egg divides to form inside eggs, which can create any cell, for example, heart, brain or T-cell. Diagram courtesy of Mike Jones and Wikipedia, modified by author.

Further Reading:

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